The present invention relates to a semiconductor device and a method of forming the same, and more particularly to a semiconductor device formed by bonding inner leads of a tape automated bonding tape or a lead frame onto electrode pads of a semiconductor chip, and a method of forming the same.
FIG. 1A is a fragmentary plan view illustrative of a film carrier tape to be used for a tape automated bonding process for packaging a semiconductor device. FIG. 1B is a fragmentary cross sectional elevation view illustrative of the film carrier tape of FIG. 1A. A polyimide base film 6 has two side regions, each of which has an alignment of sprocket holes 7 for carrying and positioning the polyimide base film 6. A longitudinal center region of the polyimide base film 6 has a square device hole 10 for allowing a semiconductor chip 3 to enter into the device hole 10. A metal foil such as a copper foil is formed on a surface of the polyimide base film 6 before a photo-lithography is carried out to pattern the metal foil thereby to form leads 2 and test pads 11. Each of the leads 2 comprises an inner lead 2A projecting over the device hole 10 and an outer lead 2B connected with the inner lead and positioned outside the device hole 10. A metal film such as gold, tin or solder is plated on the foil. Outer lead holes 9 are formed in the base film 6 so that the outer lead holes 9 are positioned under the outer leads 2B and outside the device hole 10 but inside the test pads 11. A suspender 8 is formed for suspending the leads 2 and is positioned between the outer lead holes 9 and the device hole 10.
For bonding the inner leads 2A to the semiconductor chip 3, bumps 12 have previously been provided so that the inner leads 2A are pressed onto the bumps 12 while heating the same for compression bonding or eutectic bonding to bond the metal plating of the inner leads 2A onto the bumps 12. This bonding process is known as inner lead bonding.
It has been proposed to carry out the inner lead bonding without, however, using the bumps. FIG. 2A is a fragmentary cross sectional elevation view illustrative of a conventional bump-free inner lead bonding process for bonding metal platings of the inner leads onto electrode pads of a semiconductor chip. FIG. 2B is a fragmentary cross sectional elevation view illustrative of another conventional bump-free inner lead bonding process for bonding metal platings of the inner leads onto electrode pads of a semiconductor chip. The inner lead 2 remains directly in contact with a passivation film 4 formed around a pad 5 until a bonding tool 1 presses the inner lead 2 downwardly for direct bonding of the inner lead 2 onto the pad 5.
In Japanese laid-open patent publication No. 2-119153, the bumpfree inner lead bonding is disclosed, wherein the inner lead is directly bonded onto the pad without using the bump. This publication is silent on passivation film. Considering the passivation film, the bonding tool 1 of FIG. 2A may comprise the left one of two bonding tools illustrated in FIG. 3. FIG. 3 is a view illustrative of two types of available bonding tools for inner lead bonding processes. The left one of the two types of available bonding tools in FIG. 3 has a square head portion larger than a pad opening portion. An ultrasonic wave oscillation is applied in a direction along the longitudinal direction of the inner lead for the inner lead bonding process.
In Japanese patent No. 2500725, the bump free inner lead bonding method is disclosed, which is also illustrated in FIG. 2B. The bonding tool 1 of FIG. 2B may comprise the right one of the two bonding tools illustrated in FIG. 3. The right one of the two types of available bonding tools in FIG. 3 has a cross-shaped head portion smaller than an pad opening portion of the passivation film for forming a cross-shaped depressed portion on the inner lead 2 with an ultrasonic wave oscillation.
The bump free inner lead bonding process is disadvantages in that the inner lead directly contacts the passivation film whereby the passivation film prevents the inner lead from pressing to the pad for a secure bonding. Even if the pressing force or pressure of the bonding tool is increased, a crack may appear on the passivation film or disconnection of the inner lead may appear at the comer of the head of the bonding tool.
If no passivation film is provided, then an ultrasonic wave thermocompression bonding is available for the inner lead bonding with the bumps. If, however, the passivation film is provided, then the ultrasonic wave thermocompression bonding is unavailable. The above Japanese publication describes that it is preferable that the lead is bent from a height of 1/1000 to 7/1000 of one inch. However, if the width of the bonding tool head is narrower than the width of the inner lead, then the lead is displaced not to be bonded onto the pad. Thus, a wider bonding tool head is required. In this case, however, the presence of the passivation film cause that the bonding tool head is positioned over the passivation film in the longitudinal direction of the inner lead, whereby it is difficult to carry out the inner lead bonding. The above conventional inner lead bonding method disclosed in the above Japanese publication is inapplicable when the passivation film is provided. The inner lead that is parallel to the direction of the ultrasonic wave oscillation can be pressed by the bonding tool head. The inner lead perpendicular to the direction of the ultrasonic wave oscillation cannot be pressed by the bonding tool head. Bonding is difficult because this inner lead may be displaced by the ultrasonic wave oscillation, or energy of the ultrasonic wave oscillation is hard to transfer into the inner lead. Whereas it may be proposed to rotate the semiconductor chip during the inner lead bonding process or rotate a horn for holding the bonding tool, the tape automated bonding process does not allow rotating the semiconductor chip or the horn.
In order to have settled the above problems, it was proposed to use a circle-symmetric bonding tool having a smaller head than the pad opening portion of the passivation film, so as to form a ridge-projecting portion for the inner lead bonding. Since the head of the bonding tool is smaller than the pad opening portion of the passivation film and further the circle-symmetric formation allows the inner lead to be displaced in the bonding process. If the ultrasonic wave is applied, then the inner lead is forced out. If the inner lead width is 50 micrometers or more, then the bonding is possible. If the bonding tool with the smaller head than the pad opening portion of the passivation film is used, then it is impossible to carry out the bonding at a narrow pitch. If the bonding is carried out at a narrow pitch of 80 micrometers and a pad opening diameter is 65 micrometers as well as an inner lead width of 40 micrometers, then the maximum width of the bonding tool head is 65 micrometers. In this case, since the accuracy of the bonder is .+-.10 micrometers, the bonding tool head may hit to the passivation film. If the bonding tool head width is 55 micrometers, then a displacement of not less than 7.5 micrometers of the inner lead from the bonding tool center results in an incomplete pressing by the bonding tool head onto the inner lead, whereby the application of the ultrasonic wave allows the inner lead displacement for making the inner lead bonding difficult. Tolerance of .+-.10 micrometers is actually present in the inner lead width, for which reason the inner lead bonding is difficult. In addition, a contact portion of the inner lead is the ridge-projecting portion, for which reason the bonding strength is insufficient. If, in order to prevent the displacement, the inner lead width is increased beyond the opening size of the passivation film, then the bonding tool head is restricted to deteriorate the transfer coefficient of the ultrasonic wave, thereby making the inner lead bonding difficult.
The above two conventional methods separately and individually bond each inner lead, whereby productivity is low.
Concurrent bonding to plural inner leads is disclosed in Japanese laid-open patent publication No. 8-172113. The bonding tool has a head having a plurality of rejecting portions in correspondence in position to the pad pitch. However, this method is also engaged with the above problem with displacement independently from the presence of the bumps.
In the above circumstances, it had been required to develop a novel bonding tool and a bonding method free from the above problems and disadvantages.